I'm pretty picky about how much of the padding goes down my the inside of my legs. Pearl Izumis have way too much padding for me. I just got a new pair of Sugois that have worrked out great so far on shorter rides. I'm looking forward to testing them out on a century.
I ride to work in my regular clothes everday and from that experience I can see how doing long rides with underwear on could be a bad idea. Maybe it's just my undies... but there are seams that could become irritating after a while.
Veronica